XANES spectra of Ni K-edge of Ba8Ni6Ge40, together with standard samples including Ni foil (metallic Ni0), NiIIO, LiNiIIIO2, and K2NiIV(H2IO6)2. To quantify differences in the edge positions, the K-edge energy was calculated using the integral method.[35] The resulting values were plotted in the inset of (c). The three reference compounds (i.e., NiIIO, LiNiIIIO2, and K2NiIV(H2IO6)2) were employed for linear regression (black broken line in the inset of (c)) to correlate the edge energies to oxidation states. The energy axis was calibrated by measurement of a Ni metal foil in transmission mode for the powder samples and shifting the first inflection point of the XANES rise to 8333 eV.
